Refrigeration system having liquefied refrigerant control

ABSTRACT

In a refrigeration system in the form of a refrigerant circulation circuit, a first valve is disposed within a bypass circuit of the circulation circuit for being opened to supply therethrough the hot gas outflowing from a compressor directly into an evaporator. A second valve is interposed between a condenser and the evaporator for being closed to prohibit flow of refrigerant from the condenser into the evaporator therethrough. A first relay is energized to open the second valve when a first detecting element detects finish in cooling of medium caused by thermal exchange with the evaporator. A second relay is energized to close the first valve when a second detecting element detects finish in vaporization of liquefied refrigerant accumulated within the evaporator during cooling of the medium.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a refrigeration system adapted for usein ice making machines, refrigerators or the like.

2. Discussion of the Prior Art

In the refrigeration sytem of a conventional ice making machinedisclosed in Japanese Utility Model Publication No. 60-13023, a solenoidvalue of the normally closed type is disposed within a bypass line ofthe refrigerant circulation circuit to supply therethrough the hot gasoutflowing from the refrigerant compressor directly into the evaporatorwhen the solenoid valve has been energized. Such an arrangement of thesolenoid valve is useful to dissolve the external surfaces of frozen icecubes for removal of them during the defrost cycle. It is, however,observed that during the freezing cycle prior to the defrost cycle,frost or ice is grown or formed on outer surfaces of the evaporator tolower temperature of the evaporator. The lowering in temperature of theevaporator decreases an opening degree of an expansion valve to decreasean amount of refrigerant flowing into the evaporator. As a result, flowvelocity of refrigerant within the evaporator lowers, and confinement ofliquefied refrigerant is facilitated within the evaporator. Whenaccumlated in the evaporator, the liquefied refrigerant is rapidly andconcentrically pushed out by the hot gas with high velocity and largeamount flowing out from the compressor through the solenoid valve andcirculated into the compressor. This results in shortening in life ofthe compressor and undesired noises caused by hammering the interior ofthe compressor with the circulated liquefied refrigerant.

In the case that an accumulator is disposed in a line between theevaporator and compressor to store the liquefied refrigerant flowingtherein from the evaporator, the amount of gaseous refrigerant to becirculated into the compressor will decrease in accordance with anincrease of the liquefied refrigerant in the accumulator. This resultsin deterioration of the freezing performance of the ice making machine.Furthermore, it is required that for proper restraint in an amount ofliquefied refrigerant circulated into the compressor, the accumulatorhas a large capacity. However, such an arrangement of the accumulatorresults in an increase of manufacturing cost of the refrigerationsystem.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of the present invention to providean improved refrigeration system capable of properly restrainingcirculation of liquefied refrigerant into the compressor immediatelyafter the freezing cycle without such additional component parts asdescribed above.

According to the present invention, the primary object is attained byproviding a refrigeration system in the form of a refrigerantcirculation circuit including a compressor, a condenser, an expansionvalve and an evaporator arranged for thermal exchange with medium to becooled, comprising;

a first solenoid valve disposed within a bypass circuit of therefrigerant circulation circuit to supply therethrough the hot gasoutflowing from said compressor directly into the evaporator when it hasbeen activated to be opened,

a second solenoid valve interposed between the condenser and theevaporator to prohibit flow of refrigerant from the condenser into theevaporator therethrough when it has been activated to be closed,

first detecting means for generating a first detecting signal therefromwhen detected finish in cooling of the medium caused by thermal exchangewith the evaporator,

second detecting means for generating a second detecting signaltherefrom when detected finish in vaporization of liquefied refrigerantwhich will be accumulated within the evaporator during cooling of themedium,

first activating means responsive to the first detecting signal foractivating the second solenoid valve, and

second activating means responsive to the second detecting signal foractivating the first solenoid valve.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention will bemore readily appreciated from the following detailed description ofpreferred embodiments thereof when taken together with the accompanyingdrawings, in which:

FIG. 1 illustrates a machine body of an ice making machine and arefrigeration system for the ice making machine;

FIG. 2 illustrates an electric control circuit for the ice makingmachine in accordance with the present invention;

FIG. 3 is a time chart explaining operation of various components of theice making machine;

FIG. 4 illustrates a modification of the electric control circuit shownin FIG. 2, and

FIG. 5 is a time chart explaining operation of main components of themodification.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1 and 2 of the drawings, there is illustrated anice making machine in accordance with the present invention whichcomprises a machine body B and a refrigeration system R. The machinebody B includes a water tank 10 in which water is stored, as describedlater. Within the water tank 10, an overflow pipe 11 is provided tolimit an amount of water within the water tank 10 in a predeterminedamount.

A water pump 20 is driven by an electric motor 20a to pump up water fromthe water tank 10 through an inlet 21 to supply the under pressure intoa line P₁. A leftward watering mechanism 30 has watering containers 30a,30b. The watering container 30a is supplied with the water from the lineP₁ to render the supplied water flow down along outer surfaces of a pairof ice making plates 40a, 40b of a leftward upright ice making unit 40as ice making water. The watering container 30b is supplied with waterfrom a source of water (not shown) through a line P₂ and a water valve50 in the form of a normally closed solenoid valve to render thesupplied water flow down along inner surfaces of the ice making plates40a, 40b as defrost water.

A rightward watering mechanism 60 has a watering containers 60a, 60b.The watering container 60a is supplied with the water from the line P₁to render the supplied water flow down along outer surfaces of a pair ofice making plates 70a, 70b of a rightward upright ice making unit 70.The watering container 60b is supplied with water from the source ofwater through the line P₂ and the water valve 50 to render the suppliedwater flow down along inner surfaces of the ice making plates 70a, 70bas defrost water. A perforated water plate 80 is tiltably supportedbetween the water tank 10 and the watering mechanisms 30, 60 to flow thewater from the ice making plates down into the water tank 10 throughholes 81-81. The water plate 80 receives thereon and guides ice cubes,released from the ice making units, as described later, into an icestocker (not shown).

The refrigeration system R has a refrigerant compressor 90 which isdriven by an electric motor 90a to compress a gaseous refrigerantapplied thereto from a refrigerant return line P₃ and deliver it throughan output line P₄ to a finned condenser 100 provided with a cooling fanor blower 110 driven by an electric motor 110a. The condensor 100 coolsand liquefies the refrigerant and passes it through a line P₅ to areceiver 120 which acts to separate a gaseous phase component from therefrigerant thereby to apply only the liquid phase component of therefrigerant to a line P₆.

A solenoid valve 130 of the normally closed type is selectively openedto receive the refrigerant from receiver 120 through the line P₆ so asto apply the refrigerant to an expansion valve 140 through the line P₇.The expansion valve 140 acts to expand the liquefied refrigerant throughline P₇ from the solenoid valve 130 and supply it into a line P₈. Inthis instance, the opening degree of expansion valve 140 is controlledin accordance with detecting result of a thermally detecting element 141which detects temperature of refrigerant in the line P₃. A high pressureor hot gas solenoid valve 150 of the normally closed type (hereinaftercalled as the hot gas valve 150) is disposed between hot gas bypasslines P₉, P₁₀ respectively extending from intermediate portions of thelines P₄, P₈. The hot gas valve 150 is selectively opened to supplycompressed refrigerant or hot gas under high pressure flowing outthrough bypass line P₉ from the upstream portion of line P₄ into thedownstream portion of line P₈ through bypass line P₁₀.

A leftward evaporator coil 160 is supported between the ice makingplates 40a, 40b of ice making unit 40 and connected at its upper openingto the downstream portion of the line P₈. Thus, the evaporator 160 issupplied with the expanded liquid refrigerant through the line P₈ fromthe expansion valve 140 to freeze water flowing down along the outersurfaces of ice making plates 40a, 40b. The refrigerant from theevaporator coil 160 flows into a line P₁₁. The evaporator coil 160 issupplied with hot gas from the hot gas valve 150 through the line P₁₀and the downstream portion of line P₈ and warmed by thermal exchangewith the hot gas to release frozen ice cubes therefrom. The hot gas fromevaporator coil 160 flows into the line P₁₁.

A rightward evaporator coil 170 is supported between the ice makingplate 70a, 70b of ice making unit 70. The evaporator coil 170 issupplied with the expanded refrigerant from the line P₁₁ to freeze waterflowing down along the outer surfaces of ice making plates 70a, 70b. Therefrigerant from evaporator coil 170 is circulated through return lineP₃ to the compressor 90. The evaporator coil 170 is supplied with hotgas from line P₁₁ and warmed by thermal exchange with the hot gas torelease frozen ice cubes therefrom. The hot gas from evaporator coil 170is circulated into the compressor 90.

An electric control circuit for the refrigeration system R comprises athermostat H₁ of the normally closed type which is opened at apredetermined temperature (for instance, 10° C.) in the ice stocker whenthe ice stocker is filled with ice cubes. A timer Tv has a timer switchV₁ of the normally closed type and a timer switch V₂ of the normallyopen type. The timer Tv is connected at its one terminal to one terminalof a commercially available electric power source Ps through a commonline L₁ and connected at its other terminal to the other terminal ofpower source Ps through a common line L₂ and the thermostat H₁. Thus,the timer Tv is supplied with an AC voltage through the thermostat H₁from the power source Ps to measure a predetermined water supply timeduration (for instance 3 minutes). Upon finishing measurement of thepredetermined water supply time duration, the timer Tv acts to open thetimer switch V₁ and close the timer switch V₂. The closure of the timerswitch V₂ is maintained after finish of the measurement in the timer Tvand released responsive to disconnection of the timer Tv from powersource Ps caused by opening of thermostat H₁. When maintained in itsclosed position, the timer switch V₁ supplies the AC voltage from powersource Ps through the common lines L₁, L₂ to the water and hot gasvalves 50 and 150 so as to open the same valves.

A timer Tw has a timer switch W of the normally closed type andconnected at its one terminal to the one terminal of power source Psthrough the common line L₁. The other terminal of timer Tw is connectedto the other terminal of power source Ps through a thermostat H₂ of thenormally open type, a common line L₃, the timer switch V₂, the commonline L₂ and the thermostat H₁. Thus, the timer Tw is supplied with theAC voltage from power source Ps in response to closure of the thermostatH₂ during closure of the thermostat H₁ and timer switch V₂ to measure apredetermined defrost time duration (for instance, 2 minutes). Uponfinishing measurement of the predetermined defrost time duration, thetimer Tw acts to open the timer switch W and to maintain opening oftimer switch W after finish of its measurement. The timer Tw isdisconnected from the power source Ps by opening of thermostat H₂ toclose the timer switch W. The thermostat H₂ is mounted on a portion ofline P₃ near the outlet of evaporator 70 to close when detects rise oftemperature of refrigerant within the portion of line P₃ up to apredetermined defrost temperature (for instance, 9° C.).

A relay coil Rx is associated with a relay switch X of the normally opentype to provide a relay. The relay coil Rx is connected at its one endto the one terminal of power source Ps throught the common line L₁ andconnected at its other end to the other terminal of power source Psthrough the common line L₃, timer switch V₂, common line L₂ andthermostat H₁. Thus, the relay coil Rx is energized responsive to the ACvoltage from power source Ps to close the relay switch X which suppliesthe AC voltage from the power source Ps through the thermostat H₁ to themotor 90a so as to drive it.

A relay coil Ry is associated with a relay switch Y₁ of the normallyclosed type and a relay switch Y₂ of the normally open type to provide arelay. The relay coil Ry is connected at its one end to the one terminalof power source Ps through a float switch Sf of the normally open typeand the common line L₁ and connected at its other end to the otherterminal of power source Ps through the common line L₃, timer switch V₂,common line L₂ and thermostat H₁. Thus, the relay coil Ry is energizedby the AC voltage supplied thereto from power source Ps in response toclosing of the float switch Sf during closure of the thermostat H₁ andtimer switch V₂ to open the relay switch Y₁ and to close the relayswitch Y₂. The relay switch Y₁ is conditioned in its closure to supplythe AC voltage from power source Ps through the thermostat H₁ to thesolenoid valve 130 so as to open it. Meanwhile, the relay switch Y₂holds energization of relay coil Ry in its closing during closure oftimer switch W. The float switch Sf is arranged to close when detectslowering of a level of water within the water tank 10 down to apredetermined low level. The predetermined low level defines an amountof water remained within the water tank 10 when the ice making machinehas finished freezing operation thereof.

A relay coil Rz is associated with relay switches Z₁, Z₃ of the normallyopen type and a relay switch Z₂ of the normally closed type to provide arelay. The relay coil Rz is connected at its one end to the one terminalof power source Ps through a parallel circuit of the float switch Sf anda series circuit of the relay and timer switches Y₂, W and the commonline L₁. The other end of relay coil Rz is connected to the otherterminal of power source Ps through a parallel circuit of the relayswitch Z₃ and a normally open pressure switch Sp, the common line L₃,the timer switch V₂, the common line L₂ and the thermostat H₁. Whensupplied with the AC voltage from power source Ps in response to closingof the pressure switch Sp during closure of the thermostat H₁ and thetimer and float switch V₂ and Sf, or the thermostat H₁ and the timer andrelay switch V₂, W and Y₂, the relay coil Rz is energized to close therelay switches Z₁, Z₃ and to open the relay switch Z₂.

When the relay switch Z₁ is maintained in its closed position the waterand hot gas valve 50 and 150 are energized by the AC voltage appliedthereto from power source Ps through the thermostat H₁ and timer switchV₂ to be opened. The relay switch Z₂ is conditioned in its closure tosupply the AC voltage from power source Ps to the motors 20a, 110athrough the thermostat H₁ and timer switch V₂ so as to drive them. Therelay switch Z₃ is conditioned in its closure to hold the abovementionedenergization of the relay coil Rz during closure of the relay and timerswitches Y₂, W after opening of the pressure switch Sp. The pressureswitch Sp is arranged to close when detects lowering in pressure ofrefrigerant within the line P3 down to a predetermined low pressure. Thepredetermined low pressure defines decreases in an amount of liquefiedrefrigerant within the evaporators 160, 170 down to a permissible amountwhich does not give undesired influence to the compressor 90.

Assuming that there is no ice cube within the ice stocker of the icemaking machine, the thermostat H₁ is maintained in its closure. Whensupplied with the AC voltage from power source Ps through the thermostatH₁ and the common lines L₁, L₂ at a time t=t₀ (see FIG. 3), the timer Tvstarts to measure the predetermined water supply time duration.Simultaneously, the solenoid valve 130 is supplied with the AC voltagefrom the power source Ps through the relay switch Y₁ to be opened, andthe water and hot gas valves 50 and 150 are supplied with the AC voltagefrom the power source Ps through the timer switch V₁ to be opened witheach other.

When the water valve 50 is opened, the watering containers 30b, 60b aresupplied with water from the source of water through the line P₂. Thewater from the watering containers 30b, 60b flows down along the icemaking units 40, 70 and then flows into the water tank 10 through theholes 81 of water plate 80 as ice making water. When the timer Tvfinishes measurement of the predetermined water supply time duration attime t=t₁ (see FIG. 3), the timer switch V₁ is opened, whereas the timerswitch V₂ is closed. Then, the water valve 50 is disconnected by theopened timer switch V₁ from the power source Ps to close so as to stopflow of water into the water tank 10. In addition, the hot gas valve 150is also closed together with the water valve 50.

When the timer switch V₂ is closed, the motors 20a, 110a are suppliedwith the AC voltage from the power source Ps through the thermostat H₁and relay switch Z₂ to be activated so as to drive the water pump 20 andthe cooling fan 110. Simultaneously, the relay coil Rx is supplied withthe AC voltage from power source Ps through the thermostat H₁ to beenergized so as to close the relay switch X. Thus, the motor 90a issupplied with the AC voltage from power source Ps through the thermostatH₁ and relay switch X so that it is activated to drive the compressor 90(see FIG. 3).

When the water pump 20 is driven, the water within the water tank 10 issupplied into the water containers 30a, 60a through the line P₁ and thenflows down along the outer surfaces of ice making plates of ice makingunits 40, 70 into the water tank 10 through the perforated water plate80. When the cooling fan 110 and compressor 90 are driven, the gaseousrefrigerant from line P₃ is compressed by the compressor 90 anddelivered through line P₄ to condenser 100. Then, the gaseousrefrigerant is cooled and liquefied by the condenser 100 under coolingoperation of fan 110 and supplied to the receiver 120 through line P₅.Subsequently, the receiver 120 acts to separate a gaseous phasecomponent from the refrigerant to apply only the liquid phase componentof the refrigerant through the line P₆, the solenoid valve 130 and theline P₇ to the expansion valve 140. Therefore, the expansion valve 140acts to expand the liquid refrigerant in accordance with an openingdegree given by the detecting result of the thermal sensing element 141.

Then, the evaporator 160 is supplied with the expanded refrigerant fromthe expansion valve 140 through the line P₈ to freeze the water flowingdown along the outer surfaces of ice making plates 40a, 40b of icemaking unit 40. The evaporator 170 is supplied with the refrigerant fromevaporator 160 through the line P₁₁ to freeze the water flowing downalong the outer surfaces of ice making plates 70a, 70b of ice makingunit 70. The refrigerant flowing out from the evaporator 170 iscirculated into the compressor 90 through the return line P₃. This meansthat the ice making machine is conditioned in its freezing cycle (seeFIG. 3). In addition, the timer Tw is supplied with the AC voltage fromthe power source Ps through the closed thermostats H₁, H₂ in response toclosing of the timer switch V₂ to measure the predetermined defrost timeduration so as to open the timer switch W upon finishing measurementthereof.

When the water flowing down along the outer surfaces of the ice makingplates of ice making units 40, 70 is progressively frozen by theevaporators 160, 170 into ice cubes I shown by dotted lines of FIG. 1during repetitive freezing cycles of the ice making machine, the waterlevel in water tank 10 will gradually lower to the predetermined levelat which the float switch Sf is closed (see t=t₃ in FIG. 3).Furthermore, temperature of refrigerant in line P₃ will gradually lower,resulting in opening of the thermostat H₂. Thus, the timer Tw isdisconnected from power source Ps to close the timer switch W (see t=t₂in FIG. 3).

When supplied with the AC voltage from power source Ps through thethermostat H₁ in response to closing of the float switch Sf, the relaycoil Ry is energized to open the relay switch Y₁ and to close the relayswitch Y₂ (see t=t₃ in FIG. 3). Then, the solenoid valve 130 isdisconnected from power source Ps in response to opening of the relayswitch Y₁ to be deenergized such that it is closed to prohibit flow ofrefrigerant from line P₆ to line P₇. Additionally, the relay coil Ry ismaintained in its energized condition by the closed relay and timerswitches Y₂, W.

When the flow of refrigerant from line P₆ to line P₇ is prohibited bythe solenoid valve 130, the ice making machine is conditioned in apump-down cycle or refrigerant recovery cycle (see FIG. 3). In thepump-down cycle, liquefied refrigerant accumulated in the evaporators160, 170 in accordance with lowering of the temperature of theevaporators 160, 170 during the freezing cycle lowers gradually in itspressure under operation of the compressor 90 and then is graduallyvaporized. Meanwhile, the water supplied to the containers underoperation of water pump 20 flows down continuously along the ice makingplates of ice making units 40, 70, resulting in restraint ofoverfreezing the ice making plates 40, 70 and ice cubes I. Thisfacilitates vaporization of liquefied refrigerant within the evaporators160, 170 during the pump-down cycle and release of ice cubes I duringthe following defrost cycle.

When the pressure switch Sp is closed in response to lowering inpressure of refrigerant within line P₃ caused by vaporization of theliquefied refrigerant within the evaporators 160, 170, the relay coil Rzis energized by the AC voltage supplied thereto from the power source Psthrough the thermostat H₁, timer switch V₂, relay switch Y₂ and timerswitch W to close the relay switches Z₁, Z₃ and to open the relay switchZ₂ (see t=t₄ in FIG. 3). Then, the water and hot gas valves 50, 150 aresupplied with the AC voltage from the power source Ps through thethermostat H₁ and timer switch V₂ in response to closing of the relayswitch Z₁ to be opened. This means that the ice making machine isconditioned in a defrost cycle (see FIG. 3). In this instance, themotors 20a, 110a are deactivated in response to opening of the relayswitch Z₂ to stop the water pump 20 and cooling fan 110. In addition,the energization of relay coil Rz is maintained by the closed relayswitch Z₃.

When the ice making machine is conditioned in the defrost cycle, thewatering containers 30b, 60b are supplied with water through the line P₂from the source of water by the opened water valve 50. Then, the waterfrom containers 30b, 60b flows down along the inner surfaces of the icemaking plates of ice making units 40, 70. Meanwhile, the hot gasoutflowing from compressor 90 is supplied in pressure by the opened hotgas valve 150 directly to the evaporators 160, 170 through the lines P₄,P₉, P₁₀ and the downstream portion of line P₈. Thus, the ice makingplates of units 40, 70 are warmed by thermal exchange with the waterflowing down along the ice making plates and the hot gas flowing throughthe evaporators 160, 170. This effects dissolution of the externalsurfaces of the frozen ice cubes I.

As is understood from the above description, the liquefied refrigerantwithin the evaporators 160, 170 is vapourized during the pump-down cycleprior to the defrost cycle. This reliably prevents circulation ofliquefied refrigerant from the evaporators 160, 170 into the compressor90 immediately after start of the defrost cycle. Thus, only the hot gassupplied into the evaporators 160, 170 is circulated through the line P₃into the compressor 90 during the following defrost cycle. This reliablyprevents shortening in life of the compressor 90 caused by circulationof liquefied refrigerant into the compressor 90 and also preventsundesired noises caused by hammering the interior of the compressor 90with the liquefied refrigerant circulated thereto. These effects may beattained without any arrangement of an accumulator, resulting in adecrease of manufacturing cost of the ice making machine and circulationof an enough amount of refrigerant into the compressor 90.

When the thermostat H₂ is closed at time t=t₅ (see FIG. 3) in accordancewith rise of the temperature of refrigerant within the line P₃, thetimer Tw is again supplied with the AC voltage from power source Ps tomeasure the predetermined defrost time duration, as previouslydescribed. In this instance, the float switch Sf is opened in accordancewith flow of water into the water tank 10 through the perforated waterplate 80 after start of the defrost cycle, and the pressure switch Sp isopened in accordance with flow of refrigerant in pressure into the lineP₃. When the timer switch W is opened in response to finish ofmeasurement of the timer Tw (see t=t₆ in FIG. 3), the relay coils Ry, Rzare disconnected from the power source Ps to be deenergized. Thus, thedefrost cycle is ended, and the ice cubes I released from the ice makingunits 40, 70 fall down and are guided by the water plate 80 into the icestocker.

When the relay coils Ry, Rz are deenergized the relay switch Y₁ isclosed to open the solenoid valve 130. Simultaneously, the relay switchZ₁ is opened to close the water and hot gas valves 50 and 150, and therelay switch Z₂ is closed to activate the water pump 20 and the coolingfan 110. This means that the ice making machine is again conditioned inthe freezing cycle. Therefore, the ice making machine repeats thefreezing cycle, the pump-down cycle and the defrost cycle in sequence,as previously described. When the ice stocker is filled with ice cubesI, the thermostat H₁ is opened, and the ice making machine stops inresponse to opening of the thermostat H₁.

FIG. 4 illustrates a modification of the previous embodiment which ischaracterized in that the relay coil Rz and the pressure and relayswitches Sp, Z₁, Z₂ and Z₃ described in the previous embodiment arereplaced with a timer Tu with a timer switch U₁ of the normally opentype and a timer switch U₂ of the normally closed type. The timer Tu isconnected at its one end to the common line L₃. The other end of timerTu is connected to the common line L₁ through the float switch Sf. Then,the timer Tu is supplied with the AC voltage from the common lines L₁,L₃ through the float switch Sf or the relay and timer switches Y₂, W tomeasure a predetermined measuring time duration. Upon finish ofmeasurement of the predetermined measuring time duration, the timer Tucloses the timer switch U₁, opens the timer switch U₂ and maintainsclosing of the timer switch U₁ and opening of the timer switch U₂. Thetimer switch U₁ is maintained in its closure to permit supply of the ACvoltage from power source Ps to the water and hot gas valves 50, 150through the timer switch V₂. The timer switch U₂ is maintained in itsclosure to permit supply of the AC voltage from power source Ps to themotors 20a, 110a through the timer switch V₂. In the modification, thepredetermined measuring time duration corresponds to a time durationnecessary for lowering of refrigerant pressure within the line P₃ downto the predetermined low pressure after closing of the float switch Sf.Other construction of the modification is the same as that of theprevious embodiment.

In the modification, when the float switch Sf is closed, as described inthe previous embodiment, the relay coil Ry is energized andsimultaneously the timer Tu measures the predetermined measuring timeduration (see t=t₃ in FIG. 5). When the timer Tu finishes measurementthereof at t=t₄ in a condition wherein liquefied refrigerant accumulatedwithin the evaporators 160, 170 was already vapourized during thepump-down cycle, as previously described, the timer switch U₁ is closed,whereas the timer switch U₂ is opened. Then, the water and hot gasvalves 50 and 150 are supplied with the AC voltage from power source Psin response to closing of the timer switch U₁, whereas the motors 20a,110a are deactivated in response to opening of the timer switch U₂ tostop the water pump 20 and the cooling fan 110. Thus, the ice makingmachine finishes the pump-down cycle, as previously described.

From the above description, it will be understood that the liquefiedrefrigerant accumulated within the evaporators 160, 170 are vaporized atthe pump-down cycle of the ice making machine during measurement of thetimer Tu, in case of replacement of the relay coil Rz and pressure andrelay switches Sp, Z₁, Z₂, Z₃ with the timer Tu having the timerswitches U₁, U₂. This reliably prevents circulation of liquefiedrefrigerant into the compressor 90 immediately after start of thedefrost cycle of the ice making machine.

The present invention may be adapted to various refrigeration systemhaving a hot gas valve.

In actual practices of the present invention, the solenoid valve 130 ofthe normally close type may be replaced with a solenoid valve of thenormally open type. Furthermore, the solenoid valve 130 may be alsointerposed between the downstream portion of the line P₈ and thecondenser 100. In addition, the expansion valve 140 may be replacedwith, for instance, a capillary tube,

Having now fully set forth structure and operation of preferredembodiments of the concept underlying the present invention, variousother embodiments as well as certain modifications and variations of theembodiments shown and described herein will obviously occur to thoseskilled in the art becoming familiar with the underlying concept. It isto be understood, therefore, that within the scope of the appendedclaims, the invention may be practiced otherwise than as specificallyset forth herein.

What is claimed is:
 1. A refrigeration system having a refrigerantcirculation circuit including a compressor, a condenser, an expansionvalve and an evaporator arranged for thermal exchange with medium to becooled, comprising:a first solenoid valve disposed within a bypasscircuit of said refrigerant circulation circuit to supply therethrough ahot gas outflowing from said compressor directly into said evaporatorwhen it has been activated to be opened, a second solenoid valveinterposed betweem said condenser and said evaporator to prohibit theflow of liquefied refrigerant passing therethrough from said condenserinto said evaporator when it has been activated to be closed, firstdetecting means for generating a first detecting signal therefrom whendetected finish in freezing of the medium caused by thermal exchangewith said evaporator, second detecting means for generating a seconddetecting signal therefrom when detected finish in vaporization ofliquefied refrigerant accumulated within said evaporator during freezingof the medium, first activating means responsive to the first detectingsignal for activating said second solenoid valve in a condition wherethe medium is subsequently coled by thermal exchange with saidevaporator under continuous operation of said compressor, and secondactivating means responsive to the second detecting signal foractivating said first solenoid valve immediately after deactivation ofsaid second solenoid valve.
 2. A refrigeration system as claimed inclaim 1, wherein said second solenoid valve is interposed between saidexpansion valve and a receiver arranged downstream of said condenser. 3.A refrigeration system as claimed in claim 1, wherein said seconddetecting means includes a pressure detecting element for detecting thepressure of refrigerant circulated from said evaporator into saidcompressor and for generating a second detecting signal therefrom whenthe detected pressure of refrigerant becomes lower than a predeterminedvalue during activation of said second solenoid valve, and wherein saidsecond activating means is responsive to the second detecting signalfrom said pressure detecting element for activating said first solenoidvalve.
 4. A refrigeration system as claimed in claim 1, wherein saidfirst activating means includes relay means for activating said secondsolenoid valve when energized in response to the first detecting signal,and wherein said second activating means includes relay means foractivating said first solenoid valve when energized in response to thesecond detecting signal detecting element.